Selective hydrogenation catalyst for a C3 hydrocarbon cut

ABSTRACT

A catalyst comprises an active phase constituted by palladium, and a porous support comprising at least one refractory oxide selected from the group constituted by silica, alumina and silica-alumina, in which:
         the palladium content in the catalyst is in the range 0.0025% to 1% by weight with respect to the total weight of catalyst;   at least 80% by weight of the palladium is distributed in a crust at the periphery of the porous support, the thickness of said crust being in the range 25 to 450 μm;   the specific surface area of the porous support is in the range 70 to 160 m 2 /g;   the metallic dispersion D of the palladium is less than 20%.

TECHNICAL FIELD

The selective hydrogenation process can be used to transform thepolyunsaturated compounds of oil cuts by conversion of the mostunsaturated compounds into the corresponding alkenes, avoiding completesaturation and thus the formation of the corresponding alkanes.

The aim of the invention is to propose a catalyst with improvedperformances as well as a mode for the preparation of this catalyst,with this catalyst performing very well in processes for the selectivehydrogenation of unsaturated hydrocarbon compounds present in C3hydrocarbon cuts from steam cracking and/or from catalytic cracking.

PRIOR ART

Catalysts for the selective hydrogenation of C3 cuts from steam crackingand/or catalytic cracking are generally based on palladium, in the formof small metallic particles deposited on a support which may be arefractory oxide. The palladium content and the size of the particles ofpalladium are some of the criteria which are important as regards theactivity and selectivity of the catalysts.

The macroscopic distribution of the metallic particles in the supportalso constitutes an important criterion, principally in the context ofrapid and consecutive reactions such as selective hydrogenations.Generally, these elements have to be located in a crust at the peripheryof the support in order to avoid problems with intragranular materialtransfer which could lead to defective activity and a loss ofselectivity. As an example, the document US2006/025302 describes acatalyst for the selective hydrogenation of acetylene and diolefins,comprising palladium distributed in a manner such that 90% of thepalladium is introduced into the catalyst in a crust of less than 250μm.

Furthermore, in order to improve the selectivity of selectivehydrogenation catalysts, and in particular of selective hydrogenationcatalysts for unsaturated hydrocarbon compounds present in the C3hydrocarbon cuts from steam cracking and/or catalytic cracking, it hasbeen proposed in the prior art to add a second metal selected from groupIB, preferably silver, in order to obtain bimetallic palladium-silver(Pd—Ag) catalysts. Catalysts of this type have been described in thedocuments FR 2 882 531 and FR 2 991 197. Adding silver to the activephase of the catalyst has the principal effect of reducing the metallicdispersion of the palladium but, in contrast, does not change thedistribution of the particle sizes in the catalyst.

The Applicant has surprisingly discovered that the performances of apalladium catalyst, with part of the palladium being distributed in acrust at the periphery of the support, can be significantly improved ina process for the selective hydrogenation of unsaturated hydrocarboncompounds present in the C3 hydrocarbon cuts from steam cracking and/orcatalytic cracking when said catalyst comprises an active phaseconstituted solely by palladium, i.e. it does not include any metal, inparticular silver, other than palladium in the active phase, saidcatalyst having a metallic dispersion of the palladium of less than 20%,with a palladium content and a porous support with a specific surfacearea which are well specified. Such a catalyst has been able to beobtained by a preparation process comprising a step for colloidalimpregnation as well as a specific hydrothermal treatment step, bringingabout sintering of the catalyst, having the effect of reducing themetallic dispersion of the palladium in the catalyst and unexpectedlyresulting in obtaining a catalyst with enhanced performances in terms ofselectivity.

In fact, the catalyst in accordance with the invention has a highselectivity, enabling a hydrogenation of acetylenic and/or allenicand/or diolefinic compounds to be carried out while limiting the totalhydrogenation of the mono-olefins (propane in the case of C3 cuts) andlimiting any oligomerization or polymerization reactions bringing abouta reduction in the propylene yield and early deactivation of thecatalyst.

Aims of the Invention

In a first aspect, the invention concerns a catalyst comprising anactive phase constituted by palladium, and a porous support comprisingat least one refractory oxide selected from the group constituted bysilica, alumina and silica-alumina, in which:

-   -   the palladium content in the catalyst is in the range 0.0025% to        1% by weight with respect to the total weight of catalyst;    -   at least 80% by weight of the palladium is distributed in a        crust at the periphery of the porous support, the thickness of        said crust being in the range 25 to 450 μm;    -   the specific surface area of the porous support is in the range        70 to 160 m²/g;    -   the metallic dispersion D of the palladium is less than 20%.

Preferably, the metallic dispersion D of the palladium is 18% or less.

Advantageously, the palladium content in the catalyst is in the range0.025% to 0.8% by weight with respect to the total weight of catalyst.

Preferably, the specific surface area of the porous support is in therange 70 to 150 m²/g.

Advantageously, at least 80% by weight of the palladium is distributedin a crust at the periphery of the porous support, the thickness of saidcrust being in the range 50 to 450 μm.

Preferably, the porous support is alumina.

Advantageously, the total pore volume of the support is in the range 0.1to 1.5 cm³/g.

Preferably, the porous support comprises in the range 0.0050% to 0.25%by weight of sulphur with respect to the total weight of catalyst.

Advantageously, the palladium is in the form of particles with a meansize in the range 4 to 10 nm.

In a further aspect, a process for the preparation of a catalyst inaccordance with the invention comprises the following steps:

a) preparing a colloidal suspension of palladium oxide or palladiumhydroxide in an aqueous phase by mixing an aqueous solution (I)comprising at least one hydroxide selected from the group constituted byalkali hydroxides and alkaline-earth hydroxides and an aqueous solution(II) comprising at least one precursor of palladium;b) impregnating said solution obtained from step a) onto a poroussupport comprising at least one refractory oxide selected from the groupconstituted by silica, alumina and silica-alumina;c) optionally, maturing the impregnated porous support obtained in stepb) in order to obtain a catalyst precursor;d) drying the catalyst precursor obtained in step b) or c) at atemperature in the range 70° C. to 200° C.;e) optionally, calcining the dried catalyst obtained in step d) at atemperature in the range 250° C. to 900° C.;f) carrying out a hydrothermal treatment of the dried catalyst obtainedin step d) or of the calcined catalyst obtained in step e) at atemperature in the range 500° C. to 900° C., in air comprising in therange 150 to 5000 grams of water per kg of air;g) optionally, carrying out a reduction treatment on the catalystobtained at the end of step f) by contact with a reducing gas.

Advantageously, in step a), the palladium precursor is selected from thegroup constituted by palladium chloride, palladium nitrate and palladiumsulphate.

Advantageously, in step f), a hydrothermal treatment of the driedcatalyst obtained in step d) or of the calcined catalyst obtained instep e) is carried out at a temperature in the range 600° C. to 800° C.,in air comprising 300 to 4500 grams of water per kg of air.

In a further aspect, a process for selective hydrogenation comprisesbringing a C3 cut from steam cracking and/or catalytic cracking intocontact with the catalyst in accordance with the invention or preparedin accordance with the invention, in which the temperature is in therange 0° C. to 300° C., at a pressure in the range 0.1 to 10 MPa, with amolar ratio of hydrogen/(polyunsaturated compounds to be hydrogenated)in the range 0.1 to 10 and at an hourly space velocity, HSV, in therange 0.1 to 100 h⁻¹ for a process carried out in the liquid phase, witha molar ratio of hydrogen/(polyunsaturated compounds to be hydrogenated)in the range 0.5 to 1000 and at an hourly space velocity, HSV, in therange 100 to 40000 h⁻¹ for a process carried out in the gas phase.

DETAILED DESCRIPTION OF THE INVENTION

Hereinbelow, the groups of the chemical elements are provided inaccordance with the CAS classification (CRC Handbook of Chemistry andPhysics, published by CRC press, Editor-in-chief D. R. Lide, 81^(st)edition, 2000-2001). As an example, group IB in the CAS classificationcorresponds to metals from column 11 of the new IUPAC classification.

1. Definitions

Metallic Dispersion of Particles (D)

The particle dispersion is a dimensionless quantity, often expressed asa percentage. The dispersion becomes larger as the particles becomesmaller. It is defined in the publication by R. Van Hardeveld and F.Hartog, “The statistics of surface atoms and surface sites on metalcrystals”, Surface Science 15, 1969, 189-230.

Definition of Palladium Crust Thickness

In order to analyse the distribution of the metallic phase on thesupport, a crust thickness is measured by Castaing microprobe (orelectronic microprobe microanalysis). The instrument used is a CAMECASX100, equipped with four crystal monochromators in order to analysefour elements simultaneously. The technique for analysis using theCastaing microprobe consists of detecting of X-rays emitted by a solidafter excitation of its elements using a high energy electron beam. Forthe purposes of this characterization, the grains of catalyst areembedded in epoxy resin blocks. These blocks are polished until thesection with the diameter of the beads or extrudates is obtained, thenmetallized by depositing carbon in a metallic evaporator. The electronicprobe is scanned along the diameter of five beads or extrudates in orderto obtain the mean distribution profile of the constituent elements ofthe solids.

When the palladium is distributed in the form of a crust, its localconcentration generally reduces steadily when it is measured, startingfrom the edge of the catalytic grain towards the interior. A distancefrom the grain edge at which the local palladium content becomes zerooften cannot be determined with reproducible precision. In order tomeasure a crust thickness which is significant for the majority of thepalladium particles, the crust thickness is defined as the distance tothe grain edge containing 80% by weight of the palladium.

It is defined in the publication by L. Sorbier et al. “Measurement ofpalladium crust thickness on catalyst by EPMA”, Materials Science andEngineering 32 (2012). Starting from the distribution profile obtainedusing the Castaing microprobe (c(x)), it is possible to calculate thecumulative quantity Q(y) of palladium in the grain as a function of thedistance y to the edge of a grain with radius r.

For a bead (i.e. for grains of catalyst which are not in accordance withthe invention):Q(y)=∫_(−r) ^(−y) c(x)4π·x ² dx+∫ _(y) ^(r) c(x)4π·x ² dxFor an extrudate:Q(y)=∫_(−r) ^(−r+y) c(x)2π·xdx+∫ _(r−y) ^(r) c(x)2π·xdxwherer: radius of grain;y: distance to edge of grain;x: integration variable (position on the profile).

It is assumed that the concentration profile follows the diameter fromx=−r to x=+r (x=0 being the centre).

Q(r) then corresponds to the total quantity of the element in the grain.The following equation is then solved numerically for y:

$\frac{Q(y)}{Q(r)} = 0.8$where c is a strictly positive function, Q is then a strictly increasingfunction and this equation has a unique solution which is the thicknessof the crust.

2. Catalyst

The invention concerns a catalyst comprising an active phase constitutedby palladium, and a porous support comprising at least one refractoryoxide selected from the group constituted by silica, alumina andsilica-alumina, in which:

-   -   the palladium content in the catalyst is in the range 0.0025% to        1% by weight with respect to the total weight of catalyst,        preferably in the range 0.025% to 0.8% by weight, and more        preferably in the range 0.1% to 0.75% by weight;    -   at least 80% by weight of the palladium is distributed in a        crust at the periphery of the porous support, the thickness of        said crust being in the range 100 to 450 μm, preferably in the        range 125 to 300 μm, more preferably in the range 125 to 250 μm,        and yet more preferably in the range 125 to 225 μm;    -   the specific surface area of the porous support is in the range        70 to 160 m²/g, preferably in the range 70 to 150 m²/g, and more        preferably in the range 80 to 140 m²/g;    -   the metallic dispersion D of the palladium is less than 20%,        preferably 18% or less.

The mean particle size for the palladium is in the range 4 to 10 nm,preferably in the range 3 to 6 nm. The mean crystallite size is deducedfrom measurements for the metallic dispersion of the particles (D), byapplying the dispersion-particle size relationships which are known tothe person skilled in the art and described in “Analysephysico-chimiques des catalyseurs industriels” [Physico-chemicalanalyses of industrial catalysts], Chapter I, Technip Publications,Paris, 2001.

The porous support is selected from the group constituted by silica,alumina and silica-alumina. More preferably, the support is alumina. Thealumina may be present in any of the possible crystallographic forms:alpha, delta, theta, chi, gamma, etc, alone or as a mixture. Preferably,the support is selected from alpha, delta and theta alumina. Morepreferably, alpha alumina is selected.

The specific surface area of the porous support is in the range 70 to160 m²/g, preferably in the range 70 to 150 m²/g, and more preferably inthe range 80 to 140 m²/g. The BET specific surface area is measured bynitrogen physisorption. The BET specific surface area is measured bynitrogen physisorption in accordance with the ASTM standard D3663-03 asdescribed by Rouquerol F.; Rouquerol J.; Singh K. in “Adsorption byPowders & Porous Solids: Principle, methodology and applications”,Academic Press, 1999.

The total pore volume of the support is in the range 0.1 to 1.5 cm³/g,preferably in the range 0.2 to 1.4 cm³/g, and more preferably in therange 0.25 to 1.3 cm³/g. The total pore volume is measured by mercuryporosimetry in accordance with the ASTM standard D4284-92, with awetting angle of 140°, for example using an Autopore® III modelinstrument from Microméritics®.

In one embodiment of the invention, the support for the selectivehydrogenation catalyst is purely mesoporous, i.e. it has a pore diameterin the range 2 to 50 nm, preferably in the range 5 to 30 nm and morepreferably in the range 8 to 20 nm.

In another embodiment in accordance with the invention, the selectivehydrogenation catalyst support is bimodal, the first mode beingmesoporous, i.e. with a pore diameter in the range 2 to 50 nm,preferably in the range 5 to 30 nm and more preferably in the range 8 to20 nm, and the second being macroporous, i.e. with pores with a diameterof more than 50 nm. Said support advantageously has a pore volume forpores with a pore diameter in the range 50 to 700 nm of less than 20% ofthe total pore volume of the support, preferably less than 18% of thetotal pore volume of the support and particularly preferably less than15% of the total pore volume of the support.

The support may optionally comprise sulphur. The sulphur content in thesupport may be in the range 0.0050% to 0.25% by weight with respect tothe total weight of catalyst, preferably in the range 0.0075% to 0.20%by weight.

In accordance with the invention, the porous support is in the form ofbeads, trilobes, extrudates, pellets or irregular and non-sphericalagglomerates the specific shape of which may be the result of a crushingstep. Highly advantageously, the support is in the form of beads orextrudates. Yet more advantageously, the support is in the form ofbeads. The diameter of the beads is in the range 1 mm to 10 mm,preferably in the range 2 to 8 mm, and more preferably in the range 2 to6 mm.

3. Preparation Process

The invention also concerns a process for the preparation of thecatalyst. The solution of palladium may be deposited onto the supportusing any of the techniques known to the person skilled in the art.Preferably, the palladium solution is deposited using a colloidalmethod.

More particularly, the process for the preparation of the catalyst inaccordance with the invention in general comprises the following steps:

a) preparing a colloidal suspension of palladium oxide or palladiumhydroxide in an aqueous phase by mixing an aqueous solution (I)comprising at least one hydroxide selected from the group constituted byalkali hydroxides and alkaline-earth hydroxides and an aqueous solution(II) comprising at least one precursor of palladium;b) impregnating said colloidal suspension obtained from step a) onto aporous support comprising at least one refractory oxide selected fromthe group constituted by silica, alumina and silica-alumina;c) optionally, maturing the impregnated porous support obtained in stepb) in order to obtain a catalyst precursor;d) drying the catalyst precursor obtained in step b) or c) at atemperature in the range 70° C. to 200° C.;e) optionally, calcining the dried catalyst obtained in step d) at atemperature in the range 250° C. to 900° C.;f) carrying out a hydrothermal treatment of the dried catalyst obtainedin step d) or of the calcined catalyst obtained in step e) at atemperature in the range 500° C. to 900° C., in air comprising in therange 150 to 5000 grams of water per kg of air;g) optionally, carrying out a reduction treatment on the calcinedcatalyst obtained at the end of step f) by contact with a reducing gas.

The various steps are explained in detail below.

a) Preparation of a Colloidal Suspension of Palladium Oxide or PalladiumHydroxide in an Aqueous Phase

The colloidal suspension is generally obtained by hydrolysis of thepalladium cation in an aqueous medium, which results in the formation ofparticles of palladium oxide or hydroxide in suspension.

The aqueous solution of alkali hydroxides or alkaline-earth hydroxidesis generally selected from the group constituted by aqueous solutions ofsodium hydroxide, aqueous solutions of magnesium hydroxide. Preferably,the aqueous solution is by preference an aqueous solution of sodiumhydroxide.

The palladium precursor salt is generally selected from the groupconstituted by palladium chloride, palladium nitrate and palladiumsulphate. Highly preferably, the precursor salt of palladium ispalladium nitrate.

Typically, the aqueous solution comprising at least one precursor saltof palladium [hereinafter also termed solution (II)] is supplied to asuitable apparatus, followed by the aqueous solution comprising at leastone alkali hydroxide or alkaline-earth hydroxide [hereinafter alsotermed solution (I)]. Alternatively, the solutions (I) and (II) may bepoured into the apparatus simultaneously. Preferably, the aqueoussolution (II) is poured into the apparatus, followed by the aqueoussolution (I).

The colloidal suspension generally remains in the apparatus for a dwelltime in the range 0 to 20 hours.

The concentrations of the solutions (I) and (II) are generally selectedin order to obtain a pH of the colloidal suspension in the range 1.0 to3.5. Thus, the pH of the colloidal suspension may be modified duringthis dwell time by adding quantities of acid or base compatible with thestability of the colloidal suspension.

In general, the preparation temperature is in the range 5° C. to 40° C.,and preferably in the range 15° C. to 35° C.

The concentration of palladium is preferably in the range 5 to 150millimoles per litre (mmol/L), more preferably in the range 8 to 80millimoles per litre.

b) Depositing the Colloidal Suspension Prepared in Step a) byImpregnation onto a Support, Preferably onto Alumina

The colloidal suspension prepared in step 1a) is then impregnated onto asupport.

The support may optionally undergo a series of treatments before theimpregnation step, such as calcining or hydration treatments. Thesupport may also already comprise one or more metallic elements beforeimpregnation of the colloidal suspension. Metallic elements may also beintroduced into the colloidal suspension. These metallic elements may beintroduced either by conventional techniques, or by using the process inaccordance with the present invention.

The colloidal suspension is preferably poured onto the support.Preferably, the volume of the colloidal suspension impregnated onto thesupport is in the range 0.9 to 1.1 times the pore volume. This processmay be carried out either in a batchwise manner, i.e. the step forpreparation of the colloidal suspension precedes the step forimpregnation onto the support and that the main part of the colloidalsuspension is sent to the impregnation step all at once, orcontinuously, i.e. the product obtained from step a) is sentcontinuously to step b) after adjusting the dwell time of the colloidalsuspension.

A process in which the solutions (I) and (II) are poured simultaneouslyinto a tank which continuously overflows into a zone comprising thesupport to be impregnated may be mentioned as an example of a continuousprocess.

c) Maturation of Support Impregnated During Step b) for a Period in theRange 0.5 to 40 Hours (Optional Step)

After impregnation, the impregnated support is generally matured in themoist state for 0.5 to 40 h, preferably for 1 to 30 h. Longer durationsare not excluded, but do not necessarily result in an improvement.

d) Drying the Catalyst Precursor Obtained from Step b) or c)

The catalyst precursor is generally dried in order to eliminate all or aportion of the water introduced during impregnation, preferably at atemperature in the range 50° C. to 250° C., more preferably in the range70° C. to 200° C. The drying period is in the range 0.5 h to 20 h.Longer durations are not excluded, but do not necessarily result in animprovement.

Drying is generally carried out in air from the combustion of ahydrocarbon, preferably methane, or in heated air comprising between 0and 80 grams of water per kilogram of combustion air, an oxygen contentin the range 5% to 25% by volume and a carbon dioxide content in therange 0 to 10% by volume.

e) Calcining the Dried Catalyst Obtained from Step d) in Combustion Air(Optional Step)

After drying, the catalyst may be calcined in air, preferably incombustion air, and more preferably air from the combustion of methanecomprising between 40 and 80 grams of water per kg of air, an oxygencontent in the range 5% to 15% by volume and a CO₂ content in the range4% to 10% by volume. The calcining temperature is generally in the range250° C. to 900° C., preferably in the range from approximately 300° C.to approximately 500° C. The calcining period is generally in the range0.5 h to 5 h.

f) Hydrothermal Treatment of Dried Catalyst Obtained from Step d) orCalcined Catalyst Obtained from Step e)

After the drying step d) or the calcining step e), the catalystundergoes a hydrothermal treatment in air, preferably in combustion aircomprising in the range 150 to 5000 grams of water per kg of air,preferably in the range 300 to 4500, and more preferably in the range500 to 4000 grams of water per kg of air.

The temperature of the hydrothermal treatment is in the range 500° C. to900° C., preferably in the range 600° C. to 700° C.

The duration of the hydrothermal treatment is generally in the range 0.5to 5 hours.

g) Reduction of Supported Oxide Obtained from Step f), Preferably UsingGaseous Hydrogen (Optional Step)

The catalyst is generally reduced. This step is preferably carried outin the presence of a reducing gas, either in situ, i.e. in the reactorin which the catalytic transformation is being carried out, or ex situ.Preferably, this step is carried out at a temperature in the range 80°C. to 180° C., more preferably in the range 100° C. to 160° C.

The reduction is carried out in the presence of a reducing gascomprising between 25% by volume and 100% by volume of hydrogen,preferably 100% by volume of hydrogen. The hydrogen is optionallysupplemented by a gas which is inert to reduction, preferably argon,nitrogen or methane.

The reduction generally comprises a temperature rise phase followed by aconstant temperature stage.

The duration of the constant temperature stage for reduction isgenerally in the range 1 to 10 hours, preferably in the range 2 to 8hours.

The hourly space velocity (HSV) is generally in the range 150 to 3000,preferably in the range 300 to 1500 litres of reducing gas per hour andper litre of catalyst.

Use of the Catalyst

The catalyst in accordance with the invention may be used in processesfor the selective hydrogenation of unsaturated hydrocarbon compoundspresent in the C3 hydrocarbon cuts from steam cracking and/or catalyticcracking. The selective hydrogenation process in accordance with theinvention is intended to eliminate said polyunsaturated hydrocarbonspresent in said feed to be hydrogenated without hydrogenating themono-unsaturated hydrocarbons. More particularly, the selectivehydrogenation process in accordance with the invention is intended toselectively hydrogenate propadiene and methylacetylene.

Thus, for example, the C3 steam cracking cut may have the following meancomposition: of the order of 90% by weight of propylene, of the order of3% to 8% by weight of propadiene and methylacetylene, the rest beingessentially propane.

In some C3 cuts, between 0.1% and 2% by weight of C2 and C4 may also bepresent. The specifications concerning the concentrations of thesepolyunsaturated compounds for petrochemicals and polymerization unitsare very low: 20-30 ppm by weight of MAPD (methylacetylene andpropadiene) and more than 95% of propylene for chemical qualitypropylene and less than 10 ppm by weight or even down to 1 ppm by weightfor “polymerization” quality and more than 99% propylene.

The technology used for the selective hydrogenation process involves,for example, injecting polyunsaturated hydrocarbon feed and hydrogeninto at least one fixed bed reactor as an upflow or downflow. Saidreactor may be of the isothermal or adiabatic type. An adiabatic reactoris preferred. The polyunsaturated hydrocarbon feed may advantageously bediluted with one or more re-injection(s) of effluent obtained from saidreactor in which the selective hydrogenation reaction is carried out, tovarious points of the reactor located between the inlet and outlet ofthe reactor in order to limit the temperature gradient in the reactor.The technology of the selective hydrogenation process in accordance withthe invention may also advantageously involve installing at least saidsupported catalyst in a reactive distillation column or inexchanger-reactors. The stream of hydrogen may be introduced at the sametime as the feed to be hydrogenated and/or at one or more differentpoints of the reactor.

Selective hydrogenation of the C3 cuts may be carried out in the gasphase or in the liquid phase, preferably in the liquid phase. A liquidphase reaction can be used to reduce energy costs and increase the cycletime for the catalyst.

In general, the selective hydrogenation of C3 cuts is carried out at atemperature in the range 0° C. to 300° C., preferably in the range 20°C. to 100° C., at a pressure in the range 0.1 to 10 MPa, preferably inthe range 0.5 to 5 MPa, at a molar ratio of hydrogen/(polyunsaturatedcompounds to be hydrogenated) in the range 0.1 to 10 and at an hourlyspace velocity HSV (defined as the ratio of the volume flow rate of feedto the volume of catalyst) in the range 0.1 to 100 h⁻¹ for a processcarried out in the liquid phase, with a molar ratio ofhydrogen/(polyunsaturated compounds to be hydrogenated) in the range 0.5to 1000 and at an hourly space velocity HSV in the range 100 to 40000h⁻¹ for a process carried out in the gas phase.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forthuncorrected in degrees Celsius and, all parts and percentages are byweight, unless otherwise indicated.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding application No. FR 1661622, filed Nov.29, 2016 are incorporated by reference herein.

EXAMPLES

The examples presented below are intended to demonstrate the improvementin catalytic activity for selective hydrogenation with the catalysts inaccordance with the invention. Examples 1 to 4 concern processes for thepreparation of catalysts which are not in accordance with the invention(catalysts C1 to C4), and Examples 5 and 6 concern processes for thepreparation of a catalyst in accordance with the invention (catalysts C5and C6).

Example 7 concerns the measurement of the metallic dispersion D ofcatalysts C1 to C6.

Example 8 concerns the application of these catalysts in a selectivehydrogenation reaction for a C3 cut.

These examples are presented by way of illustration and do not in anyway limit the scope of the invention.

Example 1: Preparation of a Catalyst C1, not in Accordance with theInvention

A colloidal suspension of Pd oxide was prepared, with stirring at 25°C., by diluting 1.8 g of a solution of palladium nitrate Pd(NO₃)₂containing 8.5% by weight of palladium Pd with approximately 45 mL ofdemineralized water, then adding approximately 10 mL of a sodiumhydroxide solution in order to obtain a pH of 2.4. The suspension wasthen diluted with demineralized water to a volume which corresponded tothe pore volume of the alumina support. This solution was thenimpregnated onto 80 g of an alumina with a specific surface area of 71m²/g, shaped into the form of beads. A step for maturation of theimpregnated support, before drying, for a period of 3 h was carried outin air in a confined, moist medium. The solid obtained was dried in airfor 20 h at 120° C.

The catalyst C1 was dried in air at 120° C., then calcined for 2 hoursat 450° C. in a stream of combustion air with a HSV of 500 litres ofcombustion air per litre of catalyst and per hour. The combustion aircontained approximately 60 g of water per kilogram of dry air.

The catalyst C1 prepared in this manner comprised 0.19% by weight ofpalladium with respect to the total weight of catalyst.

Characterization of catalyst C1 by Castaing microprobe showed that 80%of the Pd was distributed in a crust with a thickness of approximately222 μm.

The metallic dispersion D of the palladium of catalyst C1 was 26%.

Example 2: Preparation of a Catalyst C2, not in Accordance with theInvention

A colloidal suspension of Pd oxide was prepared, with stirring at 25°C., by diluting 1.8 g of a solution of palladium nitrate Pd(NO₃)₂containing 8.5% by weight of palladium Pd with approximately 45 mL ofdemineralized water, then adding approximately 10 mL of a sodiumhydroxide solution in order to obtain a pH of 2.4. The suspension wasthen diluted with demineralized water to a volume which corresponded tothe pore volume of the alumina support. This solution was thenimpregnated onto 80 g of an alumina with a specific surface area of 71m²/g, shaped into the form of beads. A step for maturation of theimpregnated support, before drying, for a period of 3 h was carried outin air in a confined, moist medium. The solid obtained was dried in airfor 20 h at 120° C.

The catalyst C2 was dried in air at 120° C., then calcined for 2 hoursat 650° C. in a stream of combustion air with a HSV of 4000 litres ofcombustion air per litre of catalyst and per hour. The combustion aircontained approximately 60 g of water per kilogram of dry air.

The catalyst C2 prepared in this manner comprised 1.5% by weight ofpalladium with respect to the total weight of catalyst.

Characterization of catalyst C2 by Castaing microprobe showed that 80%of the Pd was distributed in a crust with a thickness of approximately500 μm.

The metallic dispersion D of the palladium of catalyst C2 was 25%.

Example 3: Preparation of a Catalyst C3, not in Accordance with theInvention (Dry Impregnation Method)

An aqueous solution of palladium nitrate was prepared at 25° C. bydiluting 3.5 g of a solution of palladium nitrate containing 8.5% byweight of palladium with demineralized water to a volume whichcorresponded to the pore volume of the alumina support.

This solution was then impregnated (using the dry impregnation method)onto 100 grams of an alumina which had a S_(BET) of 300 m²/g. Thisalumina was in the form of beads which had a mean diameter of 3 mm.

A step for maturation of the impregnated support before drying for aperiod of 20 h was carried out in air in a confined and moist medium.The solid obtained was dried in air for 2 h at 120° C. The catalyst 1obtained was dried in air at 120° C., then calcined for 2 hours at 650°C. in a stream of combustion air with a HSV of 3000 litres of combustionair per litre of catalyst per hour. The combustion air containedapproximately 4000 g of water per kg of dry air.

The catalyst C3 prepared in this manner comprised 0.19% by weight ofpalladium with respect to the total weight of catalyst.

Characterization of catalyst C3 by Castaing microprobe showed that 80%of the Pd was distributed in a crust with a thickness of approximately500 μm.

The metallic dispersion D of the palladium of catalyst C3 was 30%.

Example 4: Preparation of a Catalyst C4, not in Accordance with theInvention

A colloidal suspension of Pd oxide was prepared, with stirring at 25°C., by diluting 1.8 g of a solution of palladium nitrate Pd(NO₃)₂containing 8.5% by weight of palladium Pd with approximately 45 mL ofdemineralized water, then adding approximately 10 mL of a sodiumhydroxide solution in order to obtain a pH of 2.4. The suspension wasthen diluted with demineralized water to a volume which corresponded tothe pore volume of the alumina support. This solution was thenimpregnated onto 80 g of an alumina with a specific surface area of 300m²/g, shaped into the form of beads. A step for maturation of theimpregnated support, before drying, for a period of 3 h was carried outin air in a confined, moist medium.

The catalyst 4 obtained was dried in air at 120° C., then calcined for 2hours at 650° C. in a stream of combustion air with a HSV of 500 litresof combustion air per litre of catalyst and per hour. The combustion aircontained approximately 4000 g of water per kg of air.

The catalyst C4 prepared in this manner comprised 0.18% by weight ofpalladium with respect to the total weight of catalyst.

Characterization of catalyst C4 by Castaing microprobe showed that 80%of the Pd was distributed in a crust with a thickness of approximately210 μm.

The metallic dispersion D of the palladium of catalyst C4 was 23%.

Example 5: Preparation of a Catalyst C5, in Accordance with theInvention

A colloidal suspension of Pd oxide was prepared, with stirring at 25°C., by diluting 1.8 g of a solution of palladium nitrate Pd(NO₃)₂containing 8.5% by weight of palladium Pd with approximately 45 mL ofdemineralized water, then adding approximately 10 mL of a sodiumhydroxide solution in order to obtain a pH of 2.4. The suspension wasthen diluted with demineralized water to a volume which corresponded tothe pore volume of the alumina support. This solution was thenimpregnated onto 80 g of an alumina with a specific surface area of 90m²/g, shaped into the form of beads. A step for maturation of theimpregnated support, before drying, for a period of 3 h was carried outin air in a confined, moist medium.

The catalyst C5 obtained was dried in air at 120° C., then calcined for2 hours at 650° C. in a stream of combustion air with a HSV of 500litres of combustion air per litre of catalyst and per hour. Thecombustion air contained approximately 3500 g of water per kg of air.

The catalyst C5 prepared in this manner comprised 0.18% by weight ofpalladium with respect to the total weight of catalyst.

Characterization of catalyst C5 by Castaing microprobe showed that 80%of the Pd was distributed in a crust with a thickness of approximately195 μm.

The metallic dispersion D of the palladium of catalyst C5 was 14%.

Example 6: Preparation of a Catalyst C6, in Accordance with theInvention

A colloidal suspension of Pd oxide was prepared, with stirring at 25°C., by diluting 1.8 g of a solution of palladium nitrate Pd(NO₃)₂containing 8.5% by weight of palladium Pd with approximately 45 mL ofdemineralized water, then adding approximately 10 mL of a sodiumhydroxide solution in order to obtain a pH of 2.4. The suspension wasthen diluted with demineralized water to a volume which corresponded tothe pore volume of the alumina support. This solution was thenimpregnated onto 80 g of an alumina with a specific surface area of 150m²/g, shaped into the form of beads. A step for maturation of theimpregnated support, before drying, for a period of 3 h was carried outin air in a confined, moist medium.

The catalyst 6 obtained was dried in air at 120° C., then calcined for 2hours at 650° C. in a stream of combustion air with a HSV of 500 litresof combustion air per litre of catalyst and per hour. The combustion aircontained approximately 3500 g of water per kg of air.

The catalyst C6 prepared in this manner comprised 0.30% by weight ofpalladium with respect to the total weight of catalyst.

Characterization of catalyst C6 by Castaing microprobe showed that 80%of the Pd was distributed in a crust with a thickness of approximately150 μm.

The metallic dispersion D of the palladium of catalyst C6 was 12%.

Example 7: Measurement of Metallic Dispersion D of Catalysts C1 to C6

The measurements of the metallic dispersions were carried out bychemisorption of carbon monoxide CO onto the catalyst which had beenreduced under 1 litre of hydrogen per hour and per gram of catalyst,with a temperature ramp-up of 300° C./h and a two hour constanttemperature stage at 150° C. The catalyst was then flushed for 1 hour at150° C. under helium then cooled to 25° C. under helium.

The CO chemisorption was carried out dynamically at 25° C. in accordancewith the usual practices known to the person skilled in the art,resulting in a volume of chemisorbed CO from which the person skilled inthe art could calculate the number of chemisorbed CO molecules. Astoichiometric ratio of one molecule of CO per atom of Pd on the surfacewas assumed in order to calculate the number of Pd atoms on the surface.The dispersion is expressed as the % of surface Pd atoms with respect toall of the Pd atoms present in the catalyst sample. The metallicdispersion D for the palladium of the catalysts C1 to C6 is presented inTable 1 below.

TABLE 1 Metallic dispersion D of catalysts C1 to C6 S_(BET) Pd Size ofsupport content Impregnation Hydrothermal Dispersion crust (m²/g) (% bywt) method treatment (%) (μm) C1 (not in 71 0.19 colloidal 450° C./60 gof 26 222 accordance) water* C2 (not in 71 1.5 colloidal 650° C./60 g of25 500 accordance) water* C3 (not in 300 0.19 Dry 650° C./4000 g 30 500accordance) impregnation of water* C4 (not in 300 0.18 colloidal 650°C./4000 g 23 210 accordance) of water* C5 (in 90 0.18 colloidal 650°C./3500 g 14 195 accordance) of water* C6 (in 150 0.30 colloidal 650°C./3500 g 12 150 accordance) of water* *per kg of dry air

Example 8: Use of Catalysts C1 to C6 for the Selective Hydrogenation ofthe C3 Steam Cracking Cut

A feed comprising 92.47% by weight of propylene, 4.12% by weight ofpropane, 1.18% by weight of methylacetylene (MA), 1.63% by weight ofpropadiene (PD) was treated with catalysts C1 to C7. Before thereaction, the selective hydrogenation catalysts were activated in astream of hydrogen at 160° C. for 2 h.

25 mL of catalyst was placed in a tube reactor in upflow mode. Thepressure was maintained at 30 bar (3 MPa) and the temperature was heldat 27° C. An hourly space velocity (HSV) of 50 h⁻¹ was applied. TheH₂/MAPD molar ratio was varied between 0, 5, 10 mol/mol. The compositionof the feed and of the effluent was measured continuously at the reactoroutlet by gas phase chromatography. The gaseous oligomers were definedas the oligomers not trapped by the various filters of the unit anddetected by the chromatographic column (composed of up to 6 carbons).The performances of the catalysts C1 to C6 are recorded in Table 2below.

TABLE 2 Selectivities measured for the selective hydrogenation of the C3cut Propylene Gaseous oligomers selectivity for a selectivity for aconversion of 90% conversion of 90% of of MAPD* MAPD* Catalyst (%) (%)C1 (not in accordance) 52 13 C2 (not in accordance) 55 14 C3 (not inaccordance) 62 18 C4 (not in accordance) 60 14 C5 (in accordance) 64 6C6 (in accordance) 71 4 *MAPD = methylacetylene and propadiene

The catalysts C5 and C6 were in accordance with the invention. They bothhad a very good selectivity for propylene and for the production ofgaseous oligomers. The most selective catalysts are those which have athin palladium crust, i.e. less than 450 microns, as well as adispersion of less than 20%.

These catalysts were prepared on supports having a specific surface areaof more than 70 m²/g and less than 160 m²/g because beyond this, theacidity of the support catalyses the oligomerization reaction, asillustrated in Example 4 in which the catalyst C4 generated a largequantity of oligomers, leading to early coking of the catalyst and to ashorter service life for the latter. Specific surface areas which aretoo high and/or a palladium content which is too high as well as COdispersions which are too high result in low performance catalysts C1 toC4, producing too much propane, a compound which is not wanted in thisapplication, and gaseous oligomer contents which are symptomatic of theproduction of more oligomers and of too much coke, thereby compromisingthe service life of the catalyst and the proper operation of thehydrogenation unit.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

The invention claimed is:
 1. A catalyst consisting of palladium, and aporous support comprising at least one refractory oxide that is silica,alumina or silica-alumina, in which: the palladium content in thecatalyst is in the range 0.0025% to 1% by weight with respect to thetotal weight of catalyst; at least 80% by weight of the palladium isdistributed in a crust at the periphery of the porous support, thethickness of said crust being in the range 125 to 225 μm; the specificsurface area of the porous support is in the range 70 to 150 m²/g; themetallic dispersion D of the palladium is less than 20%.
 2. The catalystas claimed in claim 1, in which the metallic dispersion D of thepalladium is 18% or less.
 3. The catalyst as claimed in claim 1, inwhich the palladium content in the catalyst is 0.025% to 0.8% by weightwith respect to the total weight of catalyst.
 4. The catalyst as claimedin claim 1, Therein the specific surface area of the porous support is70 to 150 m²/g.
 5. The catalyst as claimed in claim 1, wherein at least80% by weight of the palladium is distributed in a crust at theperiphery of the porous support, the thickness of said crust being 50 to450 μm.
 6. The catalyst as claimed in claim 1, wherein the poroussupport is alumina.
 7. The catalyst as claimed in claim 1, wherein thetotal pore volume of the support is in the range 0.1 to 1.5 cm³/g. 8.The catalyst as claimed in claim 1, wherein the porous support comprisesin the range 0.0050% to 0.25% by weight of sulphur with respect to thetotal weight of catalyst.
 9. The catalyst as claimed in claim 1, whereinthe palladium is in the form of particles with a mean size in the range4 to 10 nm.
 10. A process for the preparation of a catalyst as claimedin claim 1, comprising: a) preparing a colloidal suspension of palladiumoxide or palladium hydroxide in an aqueous phase by mixing an aqueoussolution (I) comprising at least one alkali hydroxide or alkaline-earthhydroxide and an aqueous solution (II) comprising at least one precursorof palladium; b) impregnating said solution obtained from a) onto aporous support comprising at least one refractory oxide that is silica,alumina or silica-alumina; c) optionally, maturing the impregnatedporous support obtained in b) in order to obtain a catalyst precursor;d) drying the catalyst precursor obtained in b) or c) at a temperaturein the range 70° C. to 200° C.; e) optionally, calcining the driedcatalyst obtained in d) at a temperature in the range 250° C. to 900°C.; f) carrying out a hydrothermal treatment of the dried catalystobtained in d) or of the calcined catalyst obtained in e) at atemperature in the range 500° C. to 900° C., in air comprising in therange 150 to 5000 grams of water per kg of air; g) optionally, carryingout a reduction treatment on the catalyst obtained at the end off) bycontact with a reducing gas.
 11. The preparation process as claimed inclaim 10, in which in a), the palladium precursor is palladium chloride,palladium nitrate or palladium sulphate.
 12. The process as claimed inclaim 10, in which in f), a hydrothermal treatment of the dried catalystobtained in d) or of the calcined catalyst obtained in e) is carried outat a temperature in the range 600° C. to 800° C., in air comprising 300to 4500 grams of water per kg of air.
 13. A process for selectivehydrogenation, comprising bringing a C3 cut from steam cracking and/orcatalytic cracking into contact with the catalyst as claimed in claim 1,in which the temperature is in the range 0° C. to 300° C., at a pressurein the range 0.1 to 10 MPa, with a molar ratio ofhydrogen/(polyunsaturated compounds to be hydrogenated) in the range 0.1to 10 and at an hourly space velocity, HSV, in the range 0.1 to 100 h⁻¹for a process carried out in the liquid phase, with a molar ratio ofhydrogen/(polyunsaturated compounds to be hydrogenated) in the range 0.5to 1000 and at an hourly space velocity, HSV, in the range 100 to 40000h⁻¹ for a process carried out in the gas phase.